Luminaires of various types, such as a direct-type back light type, an edge light type and so on, are used for liquid crystal displays in a dashboard on a vehicle, a vehicle navigation system and so on.
Low-pressure mercury lamps are widely used as luminaires, using liquid crystal, in view of the advantages of; the luminous efficacy superior to a filament lamp, the smaller heat value, the long lasting use, a larger luminous area and uniform distribution of light in view of a long current discharge channel, and so on.
The low-pressure mercury lamp is used in severe temperature conditions also. For example, when vehicles are driven in all areas and ranges of temperature, from 40.degree. C. in tropical zones, to -30.degree. C. below zero in frigid zones, the low-pressure mercury lamps used in the dashboard display, the vehicle navigation system and so on are also exposed under the above temperature conditions.
The properties of the low-pressure mercury lamp are determined by the vapor pressure of mercury sealed inside, so that they are always under the influence of the ambient temperature. Especially, the beam of light and the starting characteristic are under the influence remarkably. More specifically, the radiation of wavelengths 254 nm and 185 nm are decreased in low-temperatures, so that the beam of light is decreased and dimmed; and it is also difficult to light due to the decrease in the partial pressure of the mercury vapor in relation to the sealed inert gas, therefore taking a long time to reach the specified luminance.
The luminous efficacy reaches the maximum in the ambient temperature of approximately 40.degree. C., so that the general low-pressure mercury lamp is preferably used in the range from 5.degree. C. to 40.degree. C.
Therefore, the luminaire used in low temperature conditions is provided with a heater on the periphery of the low-pressure mercury lamp, in which the surface temperature of the low-pressure mercury lamp is controlled by various means.
As conventional luminaires, for example, there is Conventional example 1 (Japanese Patent Laid-open No. Hei7-43680), which is composed of: a heater having a predetermined width and provided on the surface of a low-pressure mercury lamp lighting a liquid crystal; a temperature detecting means for detecting a temperature of the low-pressure mercury lamp; and a controller controlling the heater in accordance with the temperature of the low-pressure mercury lamp detected by the temperature detecting means. In turn, the controller is composed of: an inverter lighting the low-pressure mercury lamp; an inverter current source connected to the inverter; and a current source controlling means for controlling On/Off of the inverter current source in accordance with the temperature of the low-pressure mercury lamp.
In Conventional example 2 (Translated National Publication of Patent Application No. Hei7-501155; International Laid-open No. WO 93/10479), plural fluorescent lamps are arranged on a liquid crystal display, a thin-section type heating element is thermally coupled onto the opposite side of the low-pressure mercury lamp from the liquid crystal display, and the heating element is thermally coupled by a PTC thermistor, therefore the temperature of the heating element is controlled by the PTC thermistor.
And further, in Conventional example 3 (Japanese Patent Laid-open No. Sho63-224140), an auto-temperature-control heating element portion having the positive temperature coefficient characteristics (PTC characteristics) is tight attached to an ordinary fluorescent lamp along a predetermined width.
However, in Conventional example 1, the heater is provided on the surface of the low-pressure mercury lamp to have the predetermined width, so that the heater shields the beam of light irradiated from the low-pressure mercury lamp, thus decreasing the amount of light irradiated onto the liquid crystal.
Furthermore, the heater is controlled by the control circuit composed of the inverter, the inverter current source and the inverter current source controlling means, so that the thermal runaway of the heater is produced when the above control circuit is not operated correctly.
In Conventional example 2, the temperature of the heating elements heating the low-pressure mercury lamp is controlled by the PTC thermistor, so that the thermal runaway of the heater is produced when the PTC thermistor is not operated correctly.
In Conventional example 3, the auto-temperature-control heating element portion is tight attached to the general fluorescent lamp along the predetermined width, so that the heating portion shields the beam of light from the fluorescent lamp.
It is an object of the present invention to provide a luminaire capable of decreasing the amount of the beam of light shielded by heating source and avoiding thermal runaway of the heating source, and a display using the luminaire.